White blood cells called T lymphocytes play critical roles in immune defense against viruses, bacteria, fungi, protozoa, and cancer cells. In the unactivated state, these cells circulate in the blood and accumulate in lymphoid tissues such as lymph nodes and spleen. Upon encounter with foreign materials (antigens) on specialized antigen presenting cells (dendritic cells), these resting T-cells become activated, undergo numerous cell divisions, and differentiate into effector cells. The effector cells leave the lymphoid tissues and blood, entering sites of infection to combat pathogens. They can also cause autoimmune pathology. After elimination of an infecting organism, most activated T-cells die, but some remain as memory cells, to provide a more rapid and vigorous response upon re-infection. Some memory T-cells recirculate in lymphoid compartments and others patrol peripheral tissues. The former provide the major source of new cells upon re-infection, whereas the latter mediate the earliest effector response to infection. Other lymphocytes such as regulatory T cells contribute to suppression of these T-cell responses. This project attempts to gain both a qualitative (especially tissue-specific 4 dimensional space and time) and a quantitative understanding of the activation, differentiation, migration, cell-cell interaction, memory status, and reactivation properties of both CD4 and CD8 T-cells. Issues such as the route, amount, and frequency of antigen exposure, as well as the presence or absence of adjuvants, are being studied for their effects on the generation and tissue distribution of effector and memory CD4 and CD8 T-cells. The movement of activated T-cells into non-lymphoid tissues is being analyzed using new imaging techniques that allow high-resolution dynamic observation of how cells migrate, interact, and carry out their effector functions. Through this research, a better understanding of lymphocyte dynamics during an immune response to infection or after vaccination or during an autoimmune response will be established. These new insights can contribute to the more effective design of vaccines and to strategies for the amelioration of autoimmune processes. We have established a robust system for vaccination using the non-replicating pox vector MVA, to permit in situ analysis of immune cell behavior in response to a clinically used vaccine vector. Variants of the virus have been developed that encode both fluorescent proteins and model antigens recognized by TCR transgenic T-cells expressing fluorescent proteins. This allows tracking of the sites of viral infection and of the location and dynamic behavior of antigen-specific T-cells during immune responses in situ, using our advanced 2-photon intravital imaging methods. Our studies indicate unexpected locations the cells initially infected by MVA within draining lymph nodes, distinct dendritic cells involvement in initial antigen presentation to CD4 vs. CD8 T-cells, a role for cross-presentation as well as direct presentation in the activation of CD8 T-cells, and different locations of naive vs. memory CD8 T-cells in the lymph node. In the case of the distinct sites of CD4 and CD8 T-cell antigen engagement, we are now trying to understand how the initial antigen activation of these two cell types on different spatially-dispersed antigen-presenting cells can be reconciled with data that indicate a requirement for antigen co-presentation to the CD8 and CD4 T-cells on the same dendritic cells for induction of robust immune memory. In terms of nave vs. memory CD8 T-cells, we have determined that these two subsets occupy very different sites within lymph nodes. Nave cells migrate primarily within the paracortical region rich in dendritic cells, whereas memory cells localize mostly to peripheral regions such as the medullary sinus and interfollicular areas. These locations are close to the entry portals of pathogens and we observe rapid movement of these already peripheral T-cells to the sinus-lining macrophage layer under guidance of CXCR3 and its cognate chemokines during infection. These data on cell localization are congruent with our recent findings of innate lymphocyte spatial pre-positioning for communication with the subcapsular sinus macrophages and our evidence for subregion-specific residence of dendritic cell subsets within the node all findings that emphasize the role of tissue micro-anatomy in effective host defense. With respect to effector function, a combination of intravital imaging and flow cytometry studies in a model of delayed-type hypersensitivity has revealed that effector CD4 T-cells in an inflamed site undergo a single round of activation to cytokine secretion status in concert with an arrest of migration, followed by a loss of cytokine production as the cells resume rapid movement in the tissue. This regain in motility occurs without the loss of antigen presentation. These data imply tuning of the antigen sensitivity of effector CD4 T-cells upon initial activation in the tissue that limits damaging cytokine production if the antigen load is not increasing, and also suggests that eventual elimination of a pathogen will depend on a constant influx of new antigen-sensitive effectors from secondary lymphoid tissue. Our data suggest this desensitization arises in part from a negative feedback loop involving PD-1, whose expression increases shortly after antigen stimulation of effector T-cells in the tissue. Engagement of PD-1 by PD-L1 that is also upregulated on antigen-presenting cells in the inflammatory tissue environment depresses signaling through the TCR and hence, desensitizes the T-cells with respect to the available antigen level. We propose that these events reflect the evolution of the adaptive immune system to a state that attempts to balance host defense against the possibility of inflammatory damage, by using antigen level as a representation of pathogen abundance and only activating effector cells to a level necessary to prevent that antigen level (pathogen load) from rising. This model also predicts that central memory cells are critical for effective eradication of many pathogens because they can supply fresh effector cells that have not yet undergone desensitization in the tissue site and hence, can continue to drive pathogen levels lower. Finally, our data suggest that the in vitro paradigm of autocrine cytokine-induced polarization of CD4+ T-cells is an oversimplification of the in vivo reality. Under suitable conditions, Th1 and Th2 effector T-cells can be generated in the absence of the canonical cytokine (IFNgamma and IL-4, respectively), or the relevant STAT protein. A key determinant of T-cell fate is the strength of signaling through the T cell antigen receptor, and in vivo studies using 2 photon imaging show the fate choice (Th1 vs. Th2) is dominated by the duration of T-cell: dendritic cell interaction and the integrated strength of signal rather than by qualitative factors such as polarizing cytokines. Our most recent data show that the duration and strength of signaling establishes both the capacity of the T-cell to receive polarizing signals at a later time and the amount of such cytokine signals produced by the associated dendritic cell. The role of adjuvants in these models appears to be primarily that of determining the costimulatory state of the presenting cell and hence, the strength of signal, and secondarily providing the polarizing cytokine that tCR signaling regulates through cytokine receptor expression checkpoints, a very different model from the one that dominates current thinking and one with important implications for determining the dose, timing, and vehicles best suited to vaccine delivery.